Hospitals, schools, prisons, call centres and the like have emergency power generators usually driven by diesel engines. The fuel for such engines must be stored onsite usually in subterranean tanks. Over time the fuel deteriorates and eventually so does the tank interior. Tank rust, fuel polymerisation, condensation and the growth of organisms in the fuel present problems which worsen as time goes by and unless the tank condition is monitored regularly the problems go unnoticed until the emergency power generators fail to start or stop after a brief run. Such tanks range in volume to 110,000 liter but typically are about 40,000 liter.
Different but related problems face fuel sellers such as service stations when they wish to stock petrol/ethyl alcohol admixture. If the fuel contains water upon delivery or water content gradually increases by condensation or unintended ingress, the water will stay miscible with the alcohol phase until it reaches about 0.5% when 40-60% of the alcohol descends to the bottom of the tank and a layer with a reduced octane rating is created. Alcohol is a polar solvent which is corrosive and loosens deposits which may otherwise have remained stable. As the temperature falls increased phase separation is noticed. These changes emerge as a result of monitoring tank condition and thereby come to the attention of the service station operator.
Mineral oil based fuels tolerate sediment and even heavily contaminated tanks may still not cause fuel filter clogging. But if those types of tanks are to switch over to holding alcohol/petrol fuels they require prior treatment. In such tanks the contamination may be 1-10% by volume.
Analysis shows that tank sludge has the following composition:
Microbes2%Rust and solid sediment5%Water11%Polymerised fuel82%
U.S. Pat. No. 5,409,025 describes a pumping apparatus specifically for filtering particulate materials from hydrocarbon fuel in underground storage tanks. Pumping and filtering the contaminated fuel is effective but the removal of particles down to a preselected size takes time and this prolongs the time when the tank is out of service.